Fix declare copyout in libgomp.oacc-c++/declare-pr94120.C

Fix declare copyout in libgomp.oacc-c++/declare-pr94120.C

Testing on the host does not make sense for 'declare copyout' for
a same-scope stack-allocated variable. Once the copyout is done,
the variable is gone. Hence, test the variable on the device. This
can be revisit after the OpenACC semantic has been fixed; but with
that fix, the test PASSes again with devices.

        PR middle-end/94120
        * testsuite/libgomp.oacc-c++/declare-pr94120.C: Fix 'declare copy(out)'
        test case.